A method or apparatus of this kind is used in local area networks, particularly in distributed systems in automation for which a precise time base is important. Automation systems are here to be understood as systems which use technical means to carry out particular operations partly or wholly according to predefined programs without human intervention. In this context, the term operations denotes e.g. manufacturing processes, assembly processes, transportation processes, quality control processes, etc. which take place particularly in an industrial environment.
Highly accurate Ethernet synchronization e.g. by means of the Precision Time Protocol (PTP) specified in IEEE 1588 requires time stamping of the Ethernet telegrams at the time of reception. Increasing the telegram length in order to add the stamp is not supported by the Media Access Controllers (MACs).
The known solutions apply the time stamp as a prefix or postfix to the Ethernet frame, which means that standard Ethernet controllers can no longer be used. This problem is solved in a proprietary manner by additional hardware which is generally connected to the Media Independent Interface (MII) between an Ethernet controller and a “Physical Layer” (PHY).
US 2003/0115369 A1 discloses a method wherein, by means of an “epoch packet” which marks the start of a frame having a plurality of time slots, a plurality of devices connected to a network are assigned said time slots together with an offset and the duration of the relevant time slot during which packets may be transmitted onto the network. The epoch packets themselves are e.g. standard Ethernet frames, the time slot information constituting to some extent the application data of the Ethernet frame. The actual time stamping for synchronization is performed here by a governor which provides the incoming epoch packets in a bus adapter of the relevant device with a time stamp after the media access controller incorporated in the adapter has removed the headers of the Ethernet frames.